Acoustic logging is often used to identify properties of formations surrounding a wellbore. As illustrated schematically in FIG. 1, a sonic tool 10 is positioned in a wellbore 16. The sonic tool 10 includes at least one transmitter or source 12 that establishes mechanical disturbances, for example using piezoelectric or magnetostrictive materials. The mechanical disturbances establish acoustic waves in the borehole fluid and the surrounding formations. The transmitter 12 may be a monopole source producing an omnidirectional pressure variation, or a dipole source that produces a directional variation. Cross dipole tools use two sets of dipoles that are oriented orthogonally. The source 12 induces several types of headwaves that propagate along the wellbore 16, including compression and flexural waves and modes such as the Stoneley mode.
The sonic tool also includes a plurality of receivers 14 that measure the wavetrain propagating in the formation and borehole fluid. The receivers may, for example, be made of piezoelectric ceramics that generate an electric current corresponding to pressure variations around the tool 10. The measured disturbances are analyzed to derive information about the velocities of the wavetrain, including the velocity of the compression wave and the fast and slow shear waves that propagate along the wellbore. The analyzed information provides insight into the structure of the formations around the wellbore 16.
Further information about the sonic logging tool may be found in:
Close, D., Cho, F., Horn, F., and Edmundson, H. (2009), “The Sound of Sonic: A Historical Perspective and Introduction to Acoustic Logging,” CSEG Recorder, Pages 34-43, May; and
Pistre, V. et al (2005), “A Modular Wireline Sonic Tool for Measurements of 3D (Azimuthal, Radial, and Axial) Formation Acoustic Properties,” SPWLA 46th Annual Logging Symposium, June 26-29, New Orleans, La.
Sedimentary rocks such as shales frequently exhibit anisotropy, which may introduce uncertainty into the acoustic analysis. It is desirable to determine accurate shear wave velocities to adequately calculate reflection coefficients and seismic reflection amplitudes. In a transversely isotropic (TI) medium, it is useful to know the shear velocity along the symmetry axis of the medium. For an inclined well in a dipping anisotropic formation, such as a shale, the shear wave velocities recorded by wireline logs will not be equal to the shear wave velocity along the symmetry axis; usually they will be larger. Even for a cross-dipole shear-log in a shale, both the fast and slow shear wave velocities may be larger than the shear wave velocity along the symmetry axis. The measured shear wave velocity can be larger than the shear wave velocity along the symmetry axis by up to 10% or more. A 10% error in the shear wave velocity can have a large effect on the calculated reflection coefficient.
One approach is to use the slow shear wave velocity recorded in a cross dipole shear sonic log. For most, if not all, rocks that exhibit intrinsic anisotropy, this shear velocity is larger than the shear velocity along the symmetry axis, but this choice may be the best choice of the two shear wave velocities measured by the shear sonic log.
There is an ongoing need for alternative and improved techniques that take the anisotropy into account.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.